In the course of looking for its transcriptional activity, the DNA region of the mouse genome containing the c-mos proto oncogene, which is homologous to the transforming gene of Moloney murine sarcoma virus (v-mos), has been characterized with respect to location and identification of repetitive DNA sequences. The c-mos gene itself appears to be a single copy gene and no other related genes have been detected by Southern blot hybridization techniques. A cluster of three members of the B1 family, a mouse repititive DNA family, was identified by Southern blot hybridization with labeled total mouse genomic DNA as a probe and by DNA sequencing. These 3 members of the B1 family are located 0.6 kb., 2.8 kb., and 5.5 kb. downstream from the putative 3 feet end of the c-mos gene. The precise location and identification of another copy of a non-B1 repetitive DNA family located between the last two B1 repeats is currently under study. This non-B1 repetitive DNA family apparently comprises 5,000-10,000 copies per diploid genome. No repetitive DNA sequences were as yet detected within 3 kb. of upstream flanking sequences of the c-mos gene by using labeled total mouse genomic DNA as a probe. A DNA clone containing the c-mos gene and about 8 kb. of 5 feet flanking region has been isolated from a genomic library. This clone will be characterized in a similar manner and by heteroduplex analysis with the analogous region of the human genes.